As a result of liberalization of the energy supply market, there has been a significant growth in interest in the provision of local distribution of energy. One means that is particularly suited to the provision of a localized energy source is the gas turbine. Such a turbine generally comprises a generator that generates electric power and a gas turbine engine for driving the generator. The turbine engine comprises a turbine rotatably mounted on a rotation shaft, a combustor for generating a combustion gas, a fuel control valve for controlling an amount of fuel supplied to the combustor, and an air compressor for compressing air supplied to the combustor.
In the gas turbine apparatus described above, fuel an amount of which is controlled by the fuel control valve and air which is compressed by the air compressor, are supplied to the combustor to form therein an air/fuel mixture. This air/fuel mixture is burnt by the combustor to generate a combustion gas for supply to the turbine, to thereby rotate it at high speed. The generator is connected to one end of the rotation shaft and is driven by the turbine through the rotation shaft to generate electric power.
In the gas turbine apparatus as described above, a variety of operation controls such as a start-up control, a constant speed operation control, and the like are performed by controlling an opening degree of the fuel control valve. For example, generation of electric power is controlled such that a detected temperature of an exhaust gas from the turbine is not allowed to exceed a predetermined value. Namely, a rise in output power from the generator is dependent on a rise in a level of combustion in a combustor. As a consequence, a maximum output power of the generator is dependent on a maximum tolerance temperature of an exhaust gas in the gas turbine engine.
FIG. 1 is a graph illustrating a relationship among an exhaust gas temperature EGT, a set power PWs and an actual output power PWa of the generator, according to a prior method for starting-up a generator. When power is required to be supplied from the generator to a load, firstly a gas turbine engine is, in general, started-up in a load-free state; under such a state, a turbine controller of a gas turbine apparatus sets, at time t1, a set output power PWs of the generator at a predetermined value. Then, in response to the set output power PWs, an opening degree of a fuel control valve is gradually increased to thereby increase an amount of fuel that is supplied, thereby causing an actual or process output power PWa from the generator to advance and an exhaust gas temperature (or process temperature) EGT to increase.
A gas turbine engine as described above is subject to an inherent tolerance maximum temperature Tmax of an exhaust gas temperature. A set output power PWs of the exhaust gas is determined so as to prevent a process or actual exhaust gas temperature EGT from the turbine reaching the tolerance maximum temperature Tmax. In a case that the exhaust gas temperature EGT does reach the set temperature Ts at the time t2, as illustrated in FIG. 1, the set power PWs of the generator is decreased at a predetermined rate. However, a case may occur wherein an actual output power PWa of the generator is significantly lower than a set output power PWs at time t2. In such a case, the actual output power PWs is controlled to further increase, with the result that the exhaust gas temperature EGT also further increases, resulting in a so-called “over-shooting” phenomenon, as shown in FIG. 1. When the set output power PWs of the generator is equal to the actual or process output power at time t3 as shown in FIG. 1, the actual output power PWs is controlled to be decreased in correspondence with the decreasing set output power PWs. Therefore, from time t3, the exhaust gas temperature EGT is changed from a positive inclination to a negative inclination, as illustrated in FIG. 1. The exhaust gas temperature EGT gradually decreases, and when it reaches the set temperature Ts at time t4, the set output power PWs of the generator is changed to increase at a predetermined rate. In response thereto, the exhaust gas temperature EGT begins to increase again. In this way, the exhaust gas temperature EGT can be substantially made to converge with the set temperature Ts, whereby the actual output power PWa can be made to substantially converge with the power PWs.
In the prior art control method of a generator described above, the actual or process output power PWa of the generator is dependent on the set temperature Ts of the exhaust gas, and the set temperature Ts is predetermined by including a margin α for a tolerance maximum temperature Tmax in view of any over-shooting phenomenon which occurs at a time of increasing the actual exhaust gas temperature. From this, it will be apparent that in the prior control method a problem arises in that an actual output power PWa is limited to a relatively low value as a consequence of a margin α of the exhaust gas temperature.
In some cases, in addition to the set temperature Ts, upper and lower set temperatures Tsu and Tsl are set, which are respectively higher and lower than the set temperature Ts. The set output power PWs is decreased at a predetermined rate when the exhaust gas temperature EGT becomes higher than a set upper temperature Tsu, while the power PWs is increased at a predetermined rate when the temperature EGT falls beneath that of the set lower temperature Tsl. From this description, it will be apparent that an exhaust gas temperature EGT reaches a set upper set temperature Tsu at a time t2 in FIG. 1, thereby setting an output power PWs at a level lower than that of a predetermined rate, to thereby reduce a temperature EGT. In this context, it should also be pointed that a temperature EGT reaches a set lower temperature Tsl at time t4, as shown in FIG. 1. As a result, the power PWs is increased at a predetermined rate to increase the temperature EGT. In this case, the exhaust gas temperature EGT can be substantially converged with the set temperature Ts, and thereby the actual output power PWa can be substantially converged with the power PWs.
However, in the prior second control method using the upper and lower set temperatures, there exists a problem which may arise as a result of increasing and decreasing rates in the set output power PWs. That is, if the changing rates are set to be relatively large, hunting phenomenon may occur at an actual output power PWa, resulting in destabilization of operation of an engine control system. On the other hand, if the rates are set to be relatively low, it may take a long time to convert the exhaust gas temperature EGT to the set temperature Ts, and “over-shooting” phenomenon may therefore readily occur.
FIG. 2 is a graph illustrating a relationship between an exhaust gas temperature EGT and a set output power PWs of a generator, in a case where increasing and decreasing rates of the set output power PWs are relatively large. As is illustrated in FIG. 2, when the exhaust gas temperature EGT reaches an upper set temperature Tsu, the set output power PWs is lowered rapidly and hence the temperature EGT can be decreased rapidly. Therefore, a problem of substantial over-shooting wherein the exhaust gas temperature EGT becomes much higher than the upper set temperature Tsu, is suppressed. However, rapid variation in the set output power PWs produces hunting at an actual output power, which in turn produces hunting on the exhaust gas temperature EGT as shown in FIG. 2. As a result, operation of the control system is destabilized.
Alternatively, an increase and decrease of the set output power PWs may occur even after the exhaust gas temperature EGT reaches the upper and lower set temperatures Tsu and Tsl, respectively. The exhaust gas temperature EGT still increases gradually by thermal inertia when the temperature EGT exceeds the upper set temperature Tsu, because the decreasing rate of the set output power PWs is small. Then, after over-shooting, the temperature EGT decreases. However, the decreasing rate of the temperature EGT is slow due to the slow rate of decrease in the set output power PWs. When the temperature EGT reaches the lower set temperature Tsl, the set output power PWs of the generator is increased again at a predetermined rate. Since the rate of increase is small, the temperature EGT continues to decrease under thermal inertia, and after over-shooting occurs it once again increases. As described above, under the condition that the increasing and decreasing rates of the set output power PWs are small, over-shooting may occur. In addition, a long period of time may be required to converge the temperature EGT and an actual output power.